1. Field of the Invention
An object of the invention is an electron emitting cathode, with which there is associated at least one control or modulation grid, to be fitted into an electronic tube of any type, notably in the field of high frequencies. The invention also concerns any electron tube with a cathode such as this.
2. Description of the Prior Art
In electron tubes of the travelling wave tube type or, more generally, in linear beam devices, the electron beams are emitted by a cathode and are controlled by at least one electrode or, most commonly, by a set of electrodes especially designed to produce and guide this set of electrons along determined trajectories.
It is necessary, at any rate, to make electrodes with a configuration such that the equipotential lines resulting therefrom in the neighbourhood of the cathode are as parallel as possible to its surface, in both off and on modes. With cathodes having large diameters as compared with the distance from the cathode to the electron beam using device, this leads to making modulation electrodes having several apertures and, therefore, taking the form of "grids".
Typically, a modulation grid, with numerous apertures for passage, uses a modulation voltage of some hundreds of volts.
The energy needed to modulate an electrode such as this is proportionate to its capacitance with respect to the cathode and its positive voltage V.sup.2.sub.ek with respect to the cathode. We can thus see the value of using low voltage electrodes, especially when the modulation frequencies are high.
However, a modulation grid that is placed in front of a surface of the cathode, emitting electrons, and that is positive with respect to this surface, receives part of the electron emission. An "interception of the beam" therefore takes place. This interception may not be troublesome when the mean density of the intercepted current is low. This is the case for medium powered or low powered devices, notably with a cathode with which a single grid is associated.
However, with high powered devices, the interception of electrons by the control grid has to be eliminated as far as possible.
It has therefore been proposed to eliminate the electron emission of the cathode in the zones facing the modulation grid, by the deposition, on the surface of the cathode, of a layer of non-emissive material. An embodiment of an approach of this type is described in the document U.S. Pat. No. 4,459,323. However, in use, it has turned out that this layer itself becomes emissive after a relatively short period, even when it is separated from the cathode by an insulating layer, following a migration of emissive material from the cathode.